Subscribe fails. No exceptions thrown; returns NULL.

[Yavor-Ivanov]

I'm running the following script on two different servers:

$subscriber = MailchimpWrapper::lists()->subscribe(
        '[REDACTED]', array('email' => $data['email']),
        null, 'html', false
);

Server A is able to subscribe users, but server B fails silently. I printed the value of $subscriber on both servers.

Server A: array(3) { ["email"]=> string(29) "test128947823873289@test5.com" ["euid"]=> string(10) "34d92c32b8" ["leid"]=> string(9) "157527861" }

Server B: NULL

This appears to be a configuration issue, but I can't figure out where the problem is. I've copied the config.php file from server A to server B just to make sure, and have run composer update on both servers.

There are no error messages in the laravel.log.